1. Field of the Invention
The present invention relates in general to toroidal type continuously variable transmissions and more particularly to a speed control device of the toroidal type continuously variable transmissions.
2. Description of the Prior Art
In toroidal type continuously variable transmissions, there have been proposed various speed control devices, some of which are described in Japanese Patent First Provisional Publications 7-198015 and 6-257661.
For clarifying the task of the present invention, the speed control devices of such publications will be outlined in the following.
The speed control device of the 7-198015 publication comprises a speed change link having a generally middle portion pivotally connected to an extension of a spool of a speed control valve. Movement of the spool controls a flow of pressurized fluid to servo-pistons by which trunnions (viz., friction roller supporting members) are actuated. One end of the speed change link is pivotally connected to an axially movable slider driven by a step motor, and the other of the speed change link carries through a ball bearing an end of an L-shaped link (viz., feedback link) which is mounted on a pivot shaft. The other end of the L-shaped link is provided with a follower member that is slidably put on an inclined surface of a precess cam. The precess cam is actuated by one of the trunnions.
In the speed control device of the 6-257661, the follower member of the L-shaped link is slidably received in an inclined groove formed in a precess cam.
When, upon receiving a command signal from a controller, the step motor is energized to move through the slider the speed change link and thus the spool of the speed control valve, the pressurized fluid is fed to selected work chambers of the servo-pistons to induce pivoting movement of the friction rollers through the trunnions. During this, the pivoting movement of the friction rollers is fed back to the speed change link through the precess cam and the L-shaped link, so that the transmission ratio defined between input and output discs is automatically controlled to a target ratio determined by the command signal.
In the speed control devices of the above-mentioned publications, to assure the feedback control, a stronger biasing spring is employed for biasing the L-shaped link in a direction to press the follower member onto the inclined surface of the precess cam as well as suppressing play of some parts associated with the speed change link.
However, usage of such stronger spring causes employment of a higher-powered and thus expensive and large-sized step motor, because when operated in a direction to pull the speed change link, the step motor has to operate against the biasing force of the spring as well as a hydraulic force fundamentally applied to the spool. Due to usage of such step motor, the speed control devices of the above-mentioned publications tend to have expensive and large-sized construction.